perf(webf): optimize flex layout and fix text invalidation regressions

integration_tests/PROFILE_HOTSPOT_TARGETS.md

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+# Profile Hotspot Targets
+
+## 2026-03-24 User Case A
+
+Source profile: `/tmp/dart_devtools_2026-03-24_14_46_17.876.json`
+
+Target stack to reproduce:
+
+- `RenderFlexLayout._layoutFlexItems` -> `RenderFlexLayout._doPerformLayout`
+- `RenderFlowLayout._layoutChildren` -> `InlineFormattingContext.layout`
+- `InlineFormattingContext._buildAndLayoutParagraph`
+- `InlineFormattingContext._layoutParagraphForConstraints`
+- `InlineFormattingContext._layoutAtomicInlineItemsForParagraph`
+- `RenderEventListener.calculateBaseline` / `RenderWidget.calculateBaseline`
+
+Supporting costs visible in the same capture:
+
+- inherited text/style getters: `fontStyle`, `fontFamily`, `wordBreak`, `textIndent`
+- render-style parent/render-box scans: `getAttachedRenderParentRenderStyle`, `getRenderBoxValueByType`
+- repeated flex run metrics and relayout work: `_tryBuildEarlyNoFlexNoStretchNoBaselineRunMetrics`, `_computeRunMetrics`, `_adjustChildrenSize`
+
+Profiler caveat:
+
+- `InlineFormattingContext._profileSection` and `Timeline.startSync` / `finishSync` currently add substantial profiler noise, so sampled CPU stacks are more reliable than leaf-only timing totals.
+
+Profiler example mapping:
+
+- `flex_inline_layout` should target this user-case stack with wrapped non-flex cards inside a flex container, rich inline content, and atomic inline controls.
+
+## 2026-03-24 User Case B
+
+Source profile: `/tmp/dart_devtools_2026-03-24_22_57_35.368.json`
+
+Target stack to reproduce:
+
+- `RenderFlexLayout._layoutFlexItems` -> `RenderFlexLayout._doPerformLayout`
+- `RenderFlexLayout._adjustChildrenSize`
+- `RenderFlexLayout._tryBuildEarlyNoFlexNoStretchNoBaselineRunMetrics`
+- `RenderFlexLayout._tryNoFlexNoStretchNoBaselineFastPath`
+- `RenderFlexLayout._computeRunMetrics`
+- `RenderFlowLayout._layoutChildren` -> `InlineFormattingContext.layout`
+- `InlineFormattingContext._buildAndLayoutParagraph`
+
+Supporting costs visible in the same capture:
+
+- paragraph follow-on work: `_layoutParagraphForConstraints`, `_layoutAtomicInlineItemsForParagraph`
+- baseline/widget work: `RenderEventListener.calculateBaseline`, `RenderWidget.performLayout`, `RenderWidget.calculateBaseline`
+- render-style scans: `getAttachedRenderParentRenderStyle`, `getRenderBoxValueByType`, `everyAttachedWidgetRenderBox`
+- value resolution: `CSSLengthValue.computedValue`
+- secondary animation load: `AnimationTimeline._onTick`
+
+Profiler caveat:
+
+- This Android trace is heavily polluted by timeline markers: `_reportTaskEvent` is about half of all leaf samples and `InlineFormattingContext._profileSection` appears in more than half of stacks.
+- `flex_inline_layout` should therefore be profiled with sampled CPU stacks, inline profile sections disabled by default, and horizontal `nowrap` no-flex cards so the `NoFlexNoStretchNoBaseline` path is exercised.